The present invention relates to a system for steering rear wheels of a motor vehicle.
There have been developed several rear-wheel steering systems for steering rear wheels by operating a rear-wheel steering actuator in response to a desired rear-wheel steering angle which is set by a control unit based on motor vehicle turn data such as a front-wheel steering force and angle, lateral acceleration, yaw rate and the like and on motor vehicle driving data in a longitudinal direction such as a vehicle speed, acceleration and deceleration.
Japanese Patent Laid-Open No. 63-125476 discloses a method to incorporate a velocity feedback control into rear-wheel steering control so as to improve response of the rear-wheel steering control when an electric motor is applied to the actuator mentioned above.
The conventional method disclosed above is to drive the rear-wheel steering actuator or the electric motor in such a manner that the control unit calculates a desired rear-wheel steering angle .delta.r by means of data regarding the front-wheel steering force and the vehicle speed, determines a desired rear-wheel steering speed .omega. in accordance with the calculated angle .delta.r and the magnitude of a deviation .epsilon. calculated by means of an actual rear-wheel steering angle .delta.r detected by a rear-wheel steering angle detecting means and drives the actuator or the motor in a direction in which .epsilon. becomes zero in response to an error .xi. of an actual rear-wheel steering speed detected by a rear-wheel steering speed detecting means from the determined .omega..
However, due to inaccurate sensors for setting the desired rear-wheel steering angle .delta.r and disturbance, .delta.r may not provide smooth steering operation with respect to actual steering operation (operation of a steering wheel). Furthermore, if the response of a control system (particularly the actuator) is worse, the deviation .epsilon. of the desired rear-wheel steering angle .delta.r from the actual rear-wheel steering angle .delta.r may be a large value. In that case, the desired rear-wheel steering speed .omega. is also set to a large value in accordance with the deviation .epsilon.. This also may not provide smooth steering with respect to actual steering operation. There is, thus, the drawback that drivers cannot have smooth steering.